Battery state of charge gauge

Abstract

A circuit for detecting and indicating the actual state of charge of a storage battery in response to signals sampled from various points in the circuit powered by the battery. Such circuit is adapted to calculate the open circuit voltage of the battery, it being realized that the open circuit voltage is directly proportional to the battery state of charge. A function generator is constructed to simulate the characteristic curve of the battery relating the internal resistance to the percent charge. Feedback is provided from the output of the indicating circuitry to the bias point of the function generator for causing the generator to produce a signal related to the actual internal resistance of the battery. The circuit is adapted to provide a reliable indication of the amount of usable energy remaining in the battery, irrespective of the conditions under which the battery was discharged.

What is claimed is: 
     
       1. Circuitry for measuring the usable energy present in an electric battery comprising, in combination, means for producing a first signal proportional to battery current, means responsive to a feedback signal for generating a second signal simulating the internal resistance of the battery, means for muliplying said first and second signals to produce a product signal, means for adding a signal related to the battery terminal voltage to said product signal thereby to produce an output signal, said output signal being coupled to said generating means to serve as said feedback signal, whereby the level of said output signal is a measure of the state of charge of the battery. 
     
     
       2. A state of charge measuring circuit for use with an electrical battery comprising in combination, means for producing a first signal proportional to current flow from the battery, means for generating a second signal proportional to the internal resistance of the battery at its actual state of charge, means for multiplying said first and second signals to produce a product signal, means for producing a third signal proportional to the loaded terminal voltage of the battery, means for adding the third signal to the product signal to produce a sum signal, means for producing a fourth signal proportional to the open circuit voltage of the battery at 0% charge, means for subtracting the fourth signal from the sum signal thereby to produce an output signal proportional to the actual state of charge of the battery. 
     
     
       3. The battery state of charge measuring circuit as set forth in claim 2 wherein the generating means includes feedback signal means coupled to said output thereby to relate said second signal to the actual state of charge of the battery. 
     
     
       4. The battery state of charge measuring circuit as set forth in claim 2 wherein the generating means comprises a function generator having a biasing input for controlling the magnitude of said second signal, and means coupling the output signal to said biasing means for feeding back the signal proportional to the state of charge of the battery thereby to relate the resistance signal to the actual state of charge of the battery. 
     
     
       5. The battery state of charge measuring circuit as set forth in claim 4 wherein the function generator comprises a linear piecewise approximation generator. 
     
     
       6. The battery state of charge measuring circuit as set forth in claim 2 further including indicator means driven by said output signal for showing the state of charge of the battery. 
     
     
       7. The battery state of charge measuring circuit as set forth in claim 2 wherein the means for producing the first signal comprises amplifier means responsive to the battery current and to a ramp signal for producing an output having a duty cycle proportional to the battery current. 
     
     
       8. The battery state of charge measuring circuit as set forth in claim 2 wherein the first signal has a duty cycle proportional to the battery current, said multiplying means comprising means for gating the second signal on and off in dependence on the duty cycle of said first signal and means for integrating the gated signal thereby to produce the product of said signals. 
     
     
       9. The battery state of charge measuring circuit as set forth in claim 2 including circuit means drawing current from said battery, wherein said means for producing a first signal comprises amplifier means coupled to said circuit means for responding to signals in said circuit means to indirectly compute said signal proportional to current flow. 
     
     
       10. The battery state of charge measuring circuit as set forth in claim 2 including circuit means drawing current from said battery, said circuit means producing a plurality of signals indicative of individual current components drawn by said circuit means, said means for producing a first signal comprising amplifier means responsive to the plurality of signals for computing the total current drawn by said circuit means. 
     
     
       11. A battery state of charge gauge for use with a battery comprising in combination, means for generating a first signal proportional to the current drawn from the battery, means for producing a ramp signal, means for combining said first signal and said ramp signal for producing a second signal having a duty cycle proportional to the current drawn from the battery, function generator means for approximating a characteristic curve relating the internal resistance of the battery to its state of charge, biasing means coupled to the function generating means for causing said function generator to produce a third signal proportional to the internal resistance of the battery at its actual state of charge, means combining the duty cycle signal and the third signal for producing the product thereof, means for generating a fourth signal proportional to the actual terminal voltage of the battery, and means for adding said third and fourth signals, thereby to produce an output signal proportional to the actual state of charge of the battery. 
     
     
       12. The battery state of charge gauge as set forth in claim 11 wherein the biasing means comprises a feedback connection coupling the output signal to the function generating means. 
     
     
       13. The battery state of charge gauge as set forth in claim 11 wherein the combining means comprises means for gating said third signal on and off in dependence upon the duty cycle of said second signal to produce a gated signal, and means for integrating the gated signal.